A machine tool of the afore-mentioned kind has been known from U.S. Pat. No. 4,932,118.
In the known machine tool, a double-spindle arrangement is to be implemented by the fact that two vertical-axis spindles are arranged next to one another in a common spindle head. The tool changer for each individual spindle of this known machine tool comprises two changer arms that, in a reciprocating manner, convey tools between a lower spindle position and an upper transfer position on a tool magazine. The transfer position is located at a lateral distance from the spindle axis, for each spindle, on the side facing away from the respective other spindle.
A relatively complicated movement sequence for the two changer arms is therefore necessary with this known arrangement, and must be asymmetrical because the changer arms are arranged symmetrically with respect to the spindle axis but both operate between a transfer position arranged outside the spindle axis and the spindle position in the spindle axis. Moreover, since both changer arms operate between the same positions, steps must be taken to ensure that as the changer arms operate in a reciprocating manner, no collision occurs between the arms and the tools that are held by grippers at the free ends of the changer arms.
For this reason, with the known machine tool it is necessary to pivot the changer arms about a plurality of axes in order to ensure the necessary movement sequence.
Although the arrangement described above can be used successfully on double-spindle machine tools, there are also applications in which this mechanical complexity is not necessary in the context of a single-spindle machine.
In another prior art machine tool, as disclosed in U.S. Pat. No. 4,683,683, the tool magazine is arranged in a U-shape in a horizontal plane (called a "horseshoe magazine"). It thus surrounds the spindle head and traveling column on both sides and at the rear. The magazine is provided, on each of the two free ends of the forward-facing arms of the U, with a transfer position for tool holders. A tool changer with two changer arms is arranged on the spindle head. One changer arm is articulated on each side of the spindle head, so that tools can be brought from one spindle of the spindle head, located below the transfer position, to one or the other transfer position. Tool changing is thus accomplished in a reciprocating manner. This means that one of the gripper arms is in each case taking out of the spindle the tool for the machining operation that has just been completed and conveying it into the one transfer position allocated to it, while the other changer arm is conveying the tool needed for the next machining operation out of the other transfer position of the tool magazine allocated to it, and into the spindle.
With this known machine tool the tools circulate in the tool magazine in a horizontal plane. A continuous path for the tools thus runs on the inner edge of the U and parallel thereto on the outer edge, and is reversed, in each case through 180 degrees, at the free ends of the arms of the U on a semicircular path.
A conveyor chain, provided with storage locations for the tools or tool holders, runs along the continuous path. The storage locations are configured as U-shaped grippers that extend outward transversely to the conveyor chain, and form a receptacle by the fact that the two arms or fingers of the U-shaped gripper engage into the standardized V-groove of the tool holder.
The arrangement is such that the tools or tool holders are held with their axis outside the lengthwise center plane of the conveyor chain. This creates two practical problems, however:
The first problem consists in the fact that the tools, held with their center of gravity outside the lengthwise center plane, are exposed to strong centrifugal forces in the outside curves of the continuous path. Particular steps must therefore be taken to retain the tools in the grippers of the conveyor chain which form the storage locations.
A second problem consists in the fact that the free ends of the gripper fingers approach one another in the inside curves of the continuous path. A certain spacing between the grippers along the conveyor chain must therefore be maintained so that the gripper fingers do not touch when passing through inside curves.
On the other hand, however, this spacing leads to a reduction in the number of tools for a given length of conveyor chain.
Machine tools of the aforesaid kind are used in large numbers in practice. Since machine tools of this kind are often acquired in order to manufacture only one specific workpiece in correspondingly large volumes over long periods of time, for example a certain component of a motor vehicle, such machine tools are often adapted to the respective specific application. It may happen in this connection that in individual cases, consideration must be given to certain additional requirements, for example additional fixtures in the operating region of the machine, or certain spatial circumstances at the machine's installation site.
It may happen in such cases that the aforesaid machine tool, because of its very compact configuration with many assemblies and accessories concentrated around the spindle head and the traveling column, does not represent the optimum solution, since a compact and compressed design of this kind naturally cannot be as easily adapted to certain circumstances.